Channel Reflections and Digital Television Signals
In the coming years digital television broadcasting will take on more and more of a dominant role in television broadcasting. Public broadcasting of digital television signals has already begun the United States. Some time in the early 2000s, it is forecasted that the broadcasting of most analog television signals will cease and will be replaced by TV signals that are digital in nature, whether over terrestrial links, i.e., over the air, cable or satellite.
A problem associated with the transmission of terrestrial digital TV signals is signal reflections. Signal reflections can be caused by many factors including stationary objects such as buildings and moving objects such as airplanes.
Weak reflections that are generated relatively close to the receiver to not pose as nearly as much of a problem as strong reflections that are generated relatively far away from the receiver. The latter type of reflections being very problematic to the reception of digital signals such as digital TV signals.
Channel reflections are another common form of linear distortion ISI which constitute a common impairment in digital communications channels. They can, however, be negated by employing an equalizer in the receiver.
In many cases the amplitude level and the time delay of the reflections are time varying. These amplitude level and the time delay variations are associated typically with wireless transmission, where they may result from movement of objects which reflect the transmitted signals. In order to compensate for this type of noise, the receiver must track the channel variations and adapt the parameters of the equalizer accordingly.